1. Field of the Invention
This invention relates to linkage mechanisms and more particularly to a mechanism for maintaining straight line motion of a platform relative to a base which is static relative thereto.
2. Description of the Related Art
Straight line mechanisms form an important class of devices used for a multitude of industrial applications. One subclass of straight line mechanisms is characterized by pivoting links (occasionally compromised by a sliding element) with the ultimate goal being exact straight line motion. With one notable exception, the Peaucellier linkage, those using only pivoting links produce only straight line approximations.
The Peaucellier mechanism is described in "Mechanisms, Linkages, and Mechanical Controls", edited by Nicholas P. Chironis, published by McGraw-Hill, Inc. 1965; and "Ingenious Mechanisms For Designers and Inventors", Vols. I and II, edited by Franklin D. Jones, The Industrial Press, 1935. It consists of eight links (including a base link), operates in a single plane, and maintains the direction of motion in a fixed and unchangeable manner.
U.S. Pat. No. 4,248,103, entitled "Straight Line Mechanism," issued to J. R. Halsall, discloses a linkage mechanism for an industrial manipulator comprising at least two straight line mechanisms arranged such that a movement in a plane of a geometric point on the linkage mechanism is affected by or affects a coordinated rotary movement of the rotatable members of the straight line mechanisms, the perimeter of the plane being determined by the straight lines, or by lines parallel thereto, along which the straight-line movement of the straight line mechanism occurs.
This mechanism requires both a sliding pair and pivoting links, including a crank, in order to produce straight line motion. Sliding pairs are a source of friction, and are relatively heavy and costly.
U.S. Pat. No. 4,651,589, entitled "Polyarticulated Retractile Mechanism", issued to M. Lambert, discloses a mechanism formed of two end-most pieces, one of which is a reference connected together by at least three legs situated in separate planes and formed from two parts, each associated, on the one hand, with the other part forming the same leg by a spherical connection and, on the other hand, with an end-most piece different from the one with which this other part is associated by means of a rotoid connection.
In the several embodiments of the Lambert invention, consisting of eight triangular links (two end panels, each with three legs, with leg sets attached to each other at three ball type swivel joints), straight line motion is not possible without coordinated manual or remote rotation of at least three independently pivotable joints connecting one end to its three legs. These three pivots each run the full length of the mated triangles. Movement of any one of the three pivots causes the end panel to tilt about an axis passing through the center of two of the three swivels. Thus, the Lambert device does not provide closed loop pivoting of links for constraining any points or points to straight line motion.
U.S. Pat. No. 4,806,068, entitled "Rotary Linear Actuator for Use in Robotic Manipulators", issued to D. Kohli et al, discloses a robotic manipulator for supporting a tool or workpiece, the manipulator including at least one rotary linear actuator adapted to be used by a base and providing for controlled movement of a manipulator link with two degrees of freedom. The robotic manipulator includes a link assembly having one end spherically joined to a movable member for supporting the movable member, the movable member being adapted to support a tool or workpiece. The link assembly is articulated, and the opposite end of the link assembly is supported by a rotary linear actuator for linear movement in a direction transverse to a longitudinal axis of the opposite end of the link assembly and for rotational movement about the axis defined by that linear movement.
Straight line motion of the Kohli et al platform is possible, but only by coordinated rotary motion of the three independent actuators. This coordination is not provided by a closed loop linkage and, therefore, a controller, such as a computer driven system of actuator sensors (including angular displacement, velocity, acceleration, etc.) must be added. Also, straight line motion of the platform, in a direction other than vertical, would be an extremely difficult task for the control system.